Override uneditable value attribute on file inputs

Opening this PR as a means of discussion. In my app I am validating the existence of a file upload, and my form can be saved and returned to by the user. When a user has already uploaded a file, I display the thumbnail under the file input, like so:

![Screen Shot 2013-03-21 at 5 22 42 PM](https://f.cloud.github.com/assets/1270317/288813/a265c800-9286-11e2-9926-c4b7fb3c675f.png)

But since there's no way to set the `value` of a file input, parsley thinks that the input is empty and throws a `data-required` validation error.

This PR adds support for a `data-file-exists` attribute, which when set to a string  and assigned to a file input, becomes the new value for that input.

Does this make sense? Is there a better (more sane) way of doing this? Thanks for your great library and any advice you can provide.